Non-magnetic substrate for magnetic disk, and magnetic disk

ABSTRACT

A non-magnetic substrate for a magnetic disk includes a substrate main body having two opposing main surfaces, and a metal film that is provided on the main surfaces and is made of a material having a loss factor of 0.01 or more. The non-magnetic substrate has a thickness (T+D) of 0.700 mm or less, the thickness (T+D) being the sum of a thickness T of the substrate main body and a thickness D of the metal film, and a ratio D/T of the thickness D of the metal film to the thickness T of the substrate main body is 0.025 or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. National stage application of International PatentApplication No. PCT/JP2018/014169, filed on Apr. 2, 2018, which, inturn, claims priority under 35 U.S.C. § 119(a) to Japanese PatentApplication No. 2017-070232, filed in Japan on Mar. 31, 2017, the entirecontents of which are hereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a non-magnetic substrate for a magneticdisk, and a magnetic disk.

Background Information

Conventionally, glass substrates and aluminum alloy substrates have beenused as magnetic-disk substrates. Magnetic-disk substrates are formed byforming a magnetic film on main surfaces of these substrates. There is ademand for magnetic disks that does not have surface defects and inwhich reading and writing of information is not hindered, and readingand writing of a large amount of information is enabled.

For example, in a case where an aluminum alloy substrate is used as anon-magnetic substrate for a magnetic disk, a surface of the aluminumalloy substrate is plated with NiP. In order to keep defects fromoccurring on the plated surface, an aluminum alloy substrate for amagnetic recording medium (may be abbreviated as an “Al alloy substrate”or an “Al—Mg alloy substrate” hereinafter) provided with a metal coatingthrough physical vapor deposition on surfaces of the substrate is known(JP 2006-302358A).

With the above-described Al alloy substrate for a magnetic recordingmedium, it is possible to reduce surface defects formed on the surfaceof this substrate plated with NiP. Accordingly, a magnetic disk in whichreading and writing of information is not hindered, and reading andwriting of a large amount of information is enabled can be provided.

However, in recent years, in the hard disk drive industry,miniaturization of magnetic particles in magnetic disks is approachingthe limit, and the speed at which recording density was improved in thepast shows signs of slowing down. On the other hand, in order to analyzebig data, there is increasing demand for an increase in the storagecapacity of hard disk drive apparatuses (may be abbreviated as HDDshereinafter). Thus, attempts have been made to increase the number ofmagnetic disks provided in one hard disk drive apparatus.

If an increase in the storage capacity is to be realized by increasingthe number of magnetic disks incorporated in a hard disk driveapparatus, there is a need to reduce the thickness of a magnetic-disksubstrate occupying the majority of the thickness of the magnetic diskin a limited space in the magnetic disk drive apparatus.

Here, it has been found that, if the thickness of the magnetic-disksubstrate is reduced, the rigidity of the substrate decreases, largevibration is likely to occur, and vibration is unlikely to settle insome cases. For example, an incredibly large number of hard disk driveapparatuses are used in a data center for a cloud, and thus hard diskdrive apparatuses are often replaced due to failures. It was found thata new hard disk drive apparatus failed by an impact occurring when thenew hard disk drive apparatus is mounted on a rack, or the period oftime until it fails is shortened. Also, more thorough studies foundthat, when a hard disk drive apparatus takes on an external impact, thehard disk drive apparatus is damaged even though the magnetic disk isnot rotating due to no power being supplied to the hard disk driveapparatus.

Unlike steady-state flutter vibration caused by the rotating magneticdisk and the air flow around the magnetic disk in a steady rotationalstate, vibration caused by an external impact in this manner attenuatesover time. However, if this vibration has a large amplitude, particlesare formed due to the magnetic head coming into contact with a rampprovided to extend over a main surface of the magnetic disk so as toretract from the magnetic disk, and a ramp member being chipped, forexample, and scratches and defects occur on surfaces of the magneticdisk in some cases. Also, if vibration does not converge, theabove-described number of instances of contact increases, and scratches,defects, and particles are more likely to occur on surfaces of themagnetic disk. In present circumstances, the magnetic-disk substrate isthick, and thus is unlikely to have an amplitude of vibration caused byan external impact that is problematic. Also, because the number ofmagnetic disks provided in a hard disk drive apparatus is small, thedistance (gap) between the magnetic disk and the ramp is relativelylarge. Thus, the magnetic disk and the ramp are unlikely to come intocontact with each other. However, in the future, if the thickness of amagnetic-disk substrate is reduced to 0.700 mm or less in order toincrease the storage capacity of a hard disk drive apparatus, forexample, vibration caused by an external impact that has conventionallynot been an issue, contact with another member accompanying vibration,and particles, scratches, recesses, and the like that are formedaccompanying contact with the ramp cannot be ignored.

SUMMARY

In view of this, an object of the present invention is to provide anon-magnetic substrate for a magnetic disk and a magnetic disk by whichvibration of the magnetic disk that is caused by an impact received fromthe outside and is different from flutter vibration can be effectivelyreduced even if the thickness of the substrate is reduced.

One aspect of the present invention is a non-magnetic substrate for amagnetic disk. The non-magnetic substrate includes

a substrate main body having two opposing main surfaces, and

a metal film that is provided on the main surfaces and is made of amaterial having a loss factor of 0.01 or more.

The non-magnetic substrate has a thickness (T+D) of 0.700 mm or less,the thickness (T+D) being the sum of a thickness T of the substrate mainbody and a thickness D of the metal film.

A ratio D/T of the thickness D of the metal film to the thickness T ofthe substrate main body is 0.025 or more.

The metal film is provided on each of the main surfaces, and is providedon an edge surface of the substrate main body.

It is preferable that the thickness of the metal film provided on eachof the main surfaces is 80% or more of the thickness of the metal filmprovided on the edge surface.

It is preferable that the non-magnetic substrate has a thickness of0.640 mm or less.

It is preferable that the non-magnetic substrate for a magnetic disk hasa disk shape, and the disk shape has an outer diameter of 90 mm or more.

It is preferable that the metal film has a Vickers hardness Hv of 100[kgf/mm²] or more.

It is preferable that the metal film is formed on the main surfaces andan outer circumferential edge surface of the substrate main body, and

the outer circumferential edge surface of the substrate main bodyforming an interface with the metal film has a surface roughness maximumheight Rz of 0.5 μm or more.

It is preferable that the metal film is formed on the main surfaces andan outer circumferential edge surface of the substrate main body, and

a surface roughness maximum height Rz of the film formed on the outercircumferential edge surface of the non-magnetic substrate for amagnetic disk is smaller than a surface roughness maximum height Rz ofthe substrate main body at the outer circumferential edge surface of thesubstrate main body.

Another aspect of the present invention is a magnetic disk, in which asurface of the non-magnetic substrate for a magnetic disk has at least amagnetic film.

According to the above-described non-magnetic substrate for a magneticdisk and magnetic disk, it is possible to effectively reduce vibrationof the magnetic disk caused by an external impact that is different fromflutter vibration even if the thickness of a magnetic-disk substrate isreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one example of an external shape of anon-magnetic substrate for a magnetic disk according to this embodiment.

FIG. 2 is a diagram showing one example of an edge portion of thenon-magnetic substrate for a magnetic disk and a film according to thisembodiment.

FIG. 3 is a diagram showing one example of vibration of the non-magneticsubstrate for a magnetic disk according to this embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a non-magnetic substrate for a magnetic disk of the presentinvention will be described in detail. Note that although the followingdescription will be given below using a magnetic-disk glass substrate, asubstrate main body of the non-magnetic substrate for a magnetic diskmay be a non-magnetic metal substrate, in addition to a glass substrate.

Aluminosilicate glass, soda-lime glass, borosilicate glass, and the likecan be used as a material of the glass substrate. In particular,amorphous aluminosilicate glass can be suitably used in that chemicalstrengthening can be performed as needed, and a magnetic-disk glasssubstrate that has excellent flatness of main surfaces and substratestrength can be produced.

For example, an aluminum alloy, a titanium alloy, Si single crystal, andthe like can be used as a material of a substrate made of metal. If analuminum alloy is used, an Al—Mg (aluminum magnesium-based) alloycontaining magnesium as a component can be used. In particular, out ofthese materials, an aluminum alloy can be suitably used.

FIG. 1 is a diagram showing an external shape of a non-magneticsubstrate for a magnetic disk according to this embodiment. As shown inFIG. 1, a non-magnetic substrate 1 for a magnetic disk according to thisembodiment (simply referred to as a “non-magnetic substrate 1”hereinafter) is a disk-shaped thin substrate provided with an inner hole2. Although there is no limitation on the size of the non-magneticsubstrate 1, the non-magnetic substrate 1 can be suitably applied to amagnetic-disk substrate having a nominal diameter of 2.5 inches, or 3.5inches, for example. In the case of a magnetic-disk substrate having anominal diameter of 3.5 inches, the disk shape preferably has an outerdiameter (diameter) of 90 mm or more. Specifically, the nominal value ofthe outer diameter of the disk shape can be set to 95 mm or 97 mm. Evenif a magnetic-disk substrate has such a large disk shape, the occurrenceof particles, scratches, and recesses caused by vibration of a magneticdisk can be reduced by forming a film, which will be described later, onthe main surfaces. Note that vibration of a magnetic disk caused by anexternal impact that is different from flutter vibration increases asthe outer diameter of the non-magnetic substrate 1 increases, and isless likely to attenuate. Thus, the non-magnetic substrate 1 accordingto this embodiment is preferably used in a magnetic disk made to a3.5-inch nominal diameter standard or better.

FIG. 2 is a diagram illustrating one example of an edge portion of thenon-magnetic substrate 1 and a film. As shown in FIG. 2, thenon-magnetic substrate 1 includes a substrate main body 3 and a film 4.

The substrate main body 3 includes a pair of main surfaces 3 a, a sidewall surface 3 b disposed in a direction orthogonal to the pair of mainsurfaces 3 a, and a pair of chamfered surfaces 3 c disposed between thepair of main surfaces 3 a and the side wall surface 3 b. The side wallsurface 3 b and the chamfered surfaces 3 c are formed at an edge portionof the non-magnetic substrate 1 on the outer circumferential side and anedge portion thereof on the inner circumferential side.

If glass is used in the substrate main body 3, for example, the glasscomposition of the substrate main body 3 may include SiO₂ in an amountof 55 to 78 mol %, Li₂O in an amount of 0.1 to 1 mol %, Na₂O in anamount of 2 to 15 mol %, and MgO, CaO, SrO, and BaO in a total amount of10 to 25 mol %, and a molar ratio (CaO/(MgO+CaO+SrO+BaO)) of the contentof CaO to the total content of MgO, CaO, SrO, and BaO may be 0.20 orless (referred to as glass 1 hereinafter).

Also, glass of the substrate main body 3 may be amorphous oxide glasscontaining SiO₂ in an amount of 45 to 68 mol %, Al₂O₃ in an amount of 5to 20 mol %, SiO₂ and Al₂O₃ in a total amount (SiO₂+Al₂O₃) of 60 to 80mol %, B₂O₃ in an amount of 0 to 5 mol %, MgO in an amount of 3 to 28mol %, CaO in an amount of 0 to 18 mol %, BaO and SrO in a total amount(BaO+SrO) of 0 to 2 mol %, alkaline earth metal oxides in a total amount(MgO+CaO+SrO+BaO) of 12 to 30 mol %, alkali metal oxides in a totalamount (Li₂O+NaO₂+K₂O) of 3.5 to 15 mol %, and at least one selectedfrom the group consisting of Sn oxides and Ce oxides in a total amountof 0.05 to 2.00 mol % (this glass is referred to as glass 2hereinafter).

As shown in FIG. 2, the main surfaces 3 a, the side wall surface 3 b,and the chamfered surfaces 3 c of the substrate main body 3 are providedwith a film 4. The film 4 improves vibration isolation properties of thenon-magnetic substrate 1.

The film 4 is a metal film made of a metal material having a loss factorof 0.01 or more. The metal material of the film 4 is a material whoseloss factor is higher than that of the material of the substrate mainbody 3. The value of a loss factor is the value at room temperature, forexample, at 25° C. Hereinafter, a loss factor is the value at roomtemperature.

Here, the loss factor of the film 4 is obtained through a vibration testin which the substrate main body 3 and the non-magnetic substrate 1obtained by forming the film 4 on the substrate main body 3 are used astest samples, and the resonance frequencies and half widths at theresonance frequencies of the test samples are obtained. In the vibrationtest, “Young's modulus, shear modulus, and internal friction measuringdevice using free resonance method (JE series)” manufactured by NihonTechno-Plus Co., Ltd. can be used, for example. The loss factors of thetest samples are obtained from resonance frequencies and half widths ofthe test samples obtained in the vibration test. Also, the loss factorof the film 4 can be calculated according to a known equation indicatedbelow, for example, from the resonance frequency and the loss factor ofthe non-magnetic substrate 1, the resonance frequency of the substratemain body 3, a ratio between the thickness of the substrate main body 3and the thickness of the film 4, and a ratio between the density of thematerial of the substrate main body 3 and the density of the metalmaterial of the film 4.

When the resonance frequency and the loss factor of the non-magneticsubstrate 1 are respectively f₁ and η₁, the resonance frequency of thesubstrate main body 3 is f₃, a ratio of the total thickness of the film4 to the thickness of the substrate main body 3 is a, and a ratio of thedensity of the metal material of the film 4 to the density of thematerial of the substrate main body 3 is b, the loss factor η₄ of thefilm 4 can be expressed as η₄=α/(α−1)·η₁ where α=(f₁/f₃)²·(1+a·b).

An Ni—P alloy containing Ni and P can be used as a material having sucha property (loss factor) of the film 4. If an Ni—P alloy is used, it issufficient to add P to an alloy to make the alloy non-magnetic. Forexample, the content of P can be set to be 5 to 15 mass %. Also, an Mgalloy, an Al—Zn alloy, an Mg—Zr alloy, and the like can be used. Notethat a sputtering method, an electroless plating method, an electrolyticplating method, or the like can be used as a film formation method. Afilm formation method need only be selected from these methods asappropriate.

When the thickness of the substrate main body 3 is a thickness T and thethickness of the film 4 is a thickness D, a thickness (T+D) of thenon-magnetic substrate 1 including the substrate main body 3 and thefilm 4 is 0.700 mm or less. A ratio D/T of the thickness D (=D1+D2) ofthe film 4 provided on a main surface 3 a to the thickness T of thesubstrate main body 3 is 0.05 or more. Note that it is preferable thatthe thickness of the film 4 does not change depending on a position on amain surface, and is constant on the main surfaces.

Although the non-magnetic substrate 1 is likely to vibrate due to anexternal impact or the like because such a non-magnetic substrate 1 hasa thickness of 0.700 mm or less, even if the above-described vibrationoccurs, the film 4 is formed on the main surface 3 a of the substratemain body 3, and thus it is possible to attenuate the vibration. Also,it is preferable that the film 4 seamlessly covers the entire metal mainbody 1 including edge surfaces thereof because the degree of suppressionof the vibration in particular increases. Also, in this case, it is morepreferable that the film 4 is harder than the substrate main body 3.Also, it is even more preferable that D1 and D2 are equal to each other.In such a case, a vibration mode in which local deformation occurs isless likely to occur, and the degree of suppression of vibration ishigher. Thus, it is possible to reduce the number of instances ofcontact with a ramp, a disk located adjacent thereto, the possibility ofcontact therewith, and an impact occurring in contact therewith.

Note that, although the films 4 are respectively formed on the mainsurfaces 3 a on both sides in this embodiment, this embodiment alsoincludes a configuration in which the film 4 is formed only on one ofthe main surfaces 3 a. In this case, the thickness D of the film 4 isthe thickness of the film 4 formed on the one main surface 3 a.

A magnetic disk produced by forming a magnetic film on the non-magneticsubstrate 1 is fixed to a spindle of a hard disk drive apparatus, in theinner hole 2 in the hard disk drive apparatus. For example, when a newhard disk drive apparatus is mounted on a rack for replacement, a harddisk drive apparatus on a rack is detached therefrom in order to movethe hard disk drive apparatus to another position, for example, the harddisk drive apparatus may be subjected to an external impact accompanyingthese operations. With the non-magnetic substrate 1 to which the innerhole 2 is fixed by the spindle, due to such an impact, vibration bywhich the main surfaces 3 a around the inner hole 2 shift in the normaldirection (the thickness direction of the non-magnetic substrate 1) ofthe main surfaces 3 a occurs. Unlike steady-state flutter vibrationcaused by the rotating magnetic disk and the air flow around themagnetic disk in a steady rotational state, as shown in FIG. 3, thisvibration is vibration that attenuates over time. FIG. 3 is a diagramillustrating one example of vibration of the non-magnetic substrate 1.

Such vibration occurs even when the magnetic disk is rotating or isstatic. It is not preferable that this vibration continues for a longtime, the magnetic disk formed from the non-magnetic substrate 1repeatedly comes into contact with the ramp in the hard disk driveapparatus, and scratches and defects occur on a surface of the magneticdisk, and particles are formed as a result of the ramp member beingchipped due to this contact therewith.

However, in the non-magnetic substrate 1, a film is constituted by amaterial having a loss factor of 0.01 or more, and the ratio D/T of thethickness D (=D1+D2) of the film 4 to the thickness T of the substratemain body 3 is 0.025 or more, and thus it is possible to attenuate thevibration early. It is preferable that the film 4 has a loss factor of0.02 or more. On the other hand, although there is no particularlimitation on the upper limit of the loss factor of the film 4, amaterial having an excessively large loss factor may be a soft materialwhose crystal are likely to break. Thus, from the viewpoint that apractical metal material can be used, a material having a loss factor of0.3 or less is preferable.

If the ratio D/T is less than 0.025, the thickness D of the film 4 isnot sufficiently thick relative to the thickness T of the substrate mainbody 3, and thus it is difficult to attenuate vibration in thenon-magnetic substrate 1 early, and the film 4 cannot reduce the initialamplitude of vibration of the main surfaces 3 a. As a result of settingthe ratio D/T to be 0.025 or more, the film 4 covering the main surfaces3 a has a sufficient thickness, and thus it is possible to attenuatevibration in the non-magnetic substrate 1 early and suppress the initialamplitude of vibration. The ratio D/T is preferably 0.03 or more andmore preferably 0.04 or more. On the other hand, although there is nolimitation on the upper limit of the ratio D/T from the viewpoint of theabove-described issues, if the ratio D/T is excessively large, powerconsumed by the hard disk drive apparatus that rotates the magnetic diskmay increase due to an increase in the weight of the non-magneticsubstrate 1 as well as an increase in the material cost of the film 4,and thus the ratio D/T is preferably set to 0.15 or less, for example.

Although the film 4 can exhibit the above-described effects even if thefilm 4 is provided only on the main surfaces 3 a and is not provided onthe side wall surface 3 b and the chamfered surfaces 3 c, as shown inFIG. 2, it is preferable that the film 4 is provided on the edgesurfaces of the substrate main body 3, that is, the side wall surface 3b and the chamfered surfaces 3 c, in addition to the main surfaces 3 a.In this case, the film 4 provided on the side wall surface 3 b and thechamfered surfaces 3 c is preferably thicker than the thicknesses D1 andD2 of the films 4 respectively provided on the main surfaces 3 a.Vibration occurring in the non-magnetic substrate 1 is vibration thatshifts in the normal direction of the main surfaces 3 a, and shifts inthe normal direction of the main surfaces 3 a at the edge surfaces ofthe substrate main body 3 together with shifting of this vibration inthe normal direction of the main surfaces 3 a. As a result ofsuppressing such shifting, it is possible to suppress the amount ofshift in the normal direction of the main surfaces 3 a, that is, theamplitude of vibration, and thus it is preferable that the film 4 isalso formed on the edge surfaces of the substrate main body 3, that is,the side wall surface 3 b and the chamfered surfaces 3 c, in addition tothe main surfaces 3 a. Specifically, it is preferable that the film 4provided on the side wall surface 3 b and the chamfered surfaces 3 c isthicker than the thicknesses D1 and D2 of the films 4 respectivelyprovided on the main surfaces 3 a because the amplitude of vibration onthe main surfaces 3 a can be suppressed. In this case, the thicknessesD1 and D2 of the films 4 provided on the main surfaces 3 a arepreferably 80% or more, more preferably 85% or more, and even morepreferably 90% or more of the thickness of the film 4 provided on theside wall surface 3 b and the chamfered surfaces 3 c (edge surfaces) inthat the amplitude of vibration can be efficiently suppressed. It ispreferable that the thicknesses D1 and D2 are closer to the thickness ofthe film 4 provided on the side wall surface 3 b and the chamferedsurfaces 3 c (edge surfaces). The larger the above-described ratio isand the more even the thickness of the film 4 is on all of the surfacesof the non-magnetic substrate 1, the less likely a vibration mode inwhich local deformation is to occur, and the more easily vibration canbe suppressed.

Because the amplitude of vibration increases as the thickness of themagnetic disk is reduced, the number of instances of contact between themagnetic disk and another member in the hard disk drive apparatusincreases, and problems arise in that particles formed along withcontact therebetween and the number of defects such as scratches andrecesses of the magnetic disk increases, but the above-describedproblems are unlikely to arise even if the non-magnetic substrate 1 hasa thickness of 0.640 mm or less. The non-magnetic substrate 1 may have athickness of 0.570 mm or less, 0.52 mm or less, or 0.400 mm or less.Also, the non-magnetic substrate 1 may have a thickness of 0.635 mm,0.550 mm, 0.500 mm, or 0.380 mm, for example. From the viewpoint ofmechanical durability, the lower limit of the thickness of thenon-magnetic substrate 1 is preferably 0.2 mm or more. Although, as thethickness of the non-magnetic substrate 1 is reduced, formation ofparticles and occurrence of defects such as scratches and recesses insome cases become more of an issue in principle, this embodimentexhibits significant effects of reducing particles and defects such asscratches and recesses.

According to one embodiment, the film 4 preferably has a Vickershardness Hv of 100 [kgf/mm²] or more. As a result of increasing theVickers hardness Hv, defects such as scratches and recesses are lesslikely to occur when the magnetic disk comes into contact with a ramp inthe hard disk drive apparatus. If the Vickers hardness Hv is less than100 [kgf/mm²], when the magnetic disk comes into contact with a ramp inthe hard disk drive apparatus, defects such as scratches and recessesoccur, and the hard disk drive apparatus is likely to fail.

According to one embodiment, it is preferable that the outercircumferential edge surface of the disk-shaped substrate main body 3that forms an interface with the film 4 has a surface roughness maximumheight Rz (JIS B 0601: 2001) of 0.5 μm or more. The substrate main body3 is provided with the film 4 on the outer circumferential edge surfacethereof as well, but an image of the cross-section of the outercircumferential edge surface of the substrate main body 3 can beacquired, and the maximum height Rz can be obtained. Specifically,first, a sample with the outer circumferential edge surface exposed isproduced by, using an ion polishing method, cutting the outercircumferential edge surface of the non-magnetic substrate 1 providedwith the film 4 along a plane that passes through the center of thenon-magnetic substrate 1 and is perpendicular to the main surfaces. Withregard to this cross-section, an image of the cross-section of the outercircumferential edge surface is obtained using a scanning electronmicroscope (SEM) at a magnification of 5000, for example. A curve ofprotrusions and recesses of a surface of the substrate main body 3 thatforms the interface where the substrate main body 3 is in contact withthe film 4 is acquired from this image through binarization or visualtracing on the image of the cross-section, for example, and a regionhaving a width 20 μm located at any portion on this curve of protrusionsand recesses is extracted to obtain the maximum height Rz.

Vibration is further suppressed by the film 4 due to the interface ofthe substrate main body 3 that is in contact with the film 4 havingsurface unevenness to some extent. It is inferred that vibration issuppressed because protruding portions of the substrate main body 3 andthe film 4 enter and engage recessed portions of each other at theinterface between the substrate main body 3 and the film 4, thusincreasing adherence therebetween, and the vibration suppression effectof the film 4 affects the substrate main body 3. Although film stress,which is a factor for causing film separation, increases as a result ofmaking the film 4 thicker, by setting the above-described maximum heightRz to be 0.5 μm or more, it is also possible to prevent film separationcaused by film stress. The outer circumferential edge surface has asmaller area than the main surface and has a complicated shape, and thusfilm separation is likely to occur.

Note that, in order to further suppress the above-described vibration,the surface roughness maximum height Rz of the outer circumferentialedge surface of the substrate main body 3 is more preferably 1.0 μm ormore, and even more preferably 1.5 μm or more. On the other hand, if themaximum height Rz is excessively large, the surface roughness of thefilm 4 after the film 4 is formed (the surface roughness on the outercircumferential edge surface of the non-magnetic substrate 1) increasesaccording to surface roughness of the substrate main body 3, foreignmatter is likely to attach to the outer circumferential edge surfaceduring processing such as main surface polishing, and foreign matteralso is likely to attach to the outer circumferential edge surface ofthe magnetic disk after a magnetic film is formed, and thus there is arisk that the yields of the non-magnetic substrates 1, and the hard diskdrive apparatuses at the time of manufacture will decrease. Note that aportion of the outer circumferential edge surface whose maximum heightRz is set to 0.5 μm or more need only be at least a portion of the outercircumferential edge surface, and in order to enhance suppression of theabove-described vibration and prevention of film separation, both theside wall surface 3 b and the chamfered surfaces 3 c preferably have amaximum height Rz of 0.5 μm or more.

On the other hand, if the maximum height Rz of the main surface of thesubstrate main body 3 is too large, there is a risk that defects willform at an early stage of the formation of the film 4 and propagate, anddefects such as a recessed portion and a crack will occur on a surfaceof the film 4. Although these defects will cause corrosion and thusshould be removed, it is difficult to remove these defects becausedefects run deep, and thus have lasting effects after a magnetic film isformed to produce a magnetic disk. Thus, it is preferable that the mainsurface of the substrate main body 3 has a maximum height Rz of 1 μm orless, for example.

Also, according to one embodiment, the surface roughness maximum heightRz of the film 4 provided on the outer circumferential edge surface ofthe non-magnetic substrate 1 is preferably smaller than the surfaceroughness maximum height Rz of the substrate main body 3 at the outercircumferential edge surface of the substrate main body 3 (on aninterface surface that is in contact with the film 4).

With regard to the surface roughness maximum height Rz of the film 4 onthe outer circumferential edge surface of the non-magnetic substrate 1,for example, the maximum height Rz is obtained at a plurality ofpositions (e.g., three positions) on the outer circumferential edgesurface using a stylus surface roughness/contour shape measurementdevice under the following conditions, and an average value thereofobtained at the plurality of positions is used as the surface roughnessmaximum height Rz of the film 4. Note that the direction in which thestylus moves (scans) is the thickness direction of the non-magneticsubstrate 1.

-   -   Shape of stylus: the radius of the leading end is 2 μm, and a        taper angle of the cone is 60 degrees    -   Stylus load: 0.75 mN    -   Stylus moving speed: 0.02 mm/sec    -   Sampling length: 0.08 mm    -   Filter λc: 0.08 mm    -   Filter λs: 0.0008 mm.

If the film 4 provided on the outer circumferential edge surface of thenon-magnetic substrate 1 has an excessive large surface roughnessmaximum height Rz, foreign matter is likely to attach to thenon-magnetic substrate 1 as a result of the film 4, and thus theabove-described maximum height Rz is preferably small. The maximumheight Rz is more preferably 1.0 μm or less, and even more preferably0.5 μm or less. Note that, although a position on the outercircumferential edge surface of the non-magnetic substrate 1 at whichthe maximum height Rz is limited need only be located in at least aportion of the outer circumferential edge surface of the non-magneticsubstrate 1, in order to enhance the above-described vibrationsuppression effect and film separation prevention effect, this positionis preferably located in a corresponding portion on a surface of thenon-magnetic substrate 1 corresponding to the side wall surface 3 b andthe chamfered surfaces 3 c. Note that the surface roughness of the film4 on the outer circumferential edge surface is likely to follow surfaceroughness of the substrate main body 3, which is the underlayer. Thus,if a surface of the substrate main body 3 is too rough, additionalprocessing such as edge surface polishing processing may be requiredafter the film 4 is formed.

Such a non-magnetic substrate 1 is produced as follows, for example.Here, a case where a glass substrate is used as the non-magneticsubstrate 1 will be described as one example.

First, processing for molding a glass blank that serves as a rawmaterial of a plate-shaped magnetic-disk glass substrate having a pairof main surfaces is performed. Next, the glass blank is roughly ground.Then, shape processing and edge surface polishing are performed on theglass blank. Then, precision grinding is performed on a glass substrateobtained from the glass blank, using fixed abrasive particles. Then,first polishing, chemical strengthening, and second polishing areperformed on the glass substrate. Then, film formation and filmpolishing are performed. Note that, although the glass substrate isproduced in the above-described flow in this embodiment, it is notnecessary to always perform the above-described processes and theseprocesses may be omitted as appropriate. For example, in theabove-described processes, edge surface polishing, precision grinding,first polishing, chemical strengthening, and second polishing need notbe carried out. Hereinafter, each of the processes will be described.

(a) Molding of Glass Blank

A press molding method may be used to mold the glass blank, for example.A circular glass blank can be obtained using a press molding method.Also, a glass blank can be manufactured using a known manufacturingmethod such as a downdraw method, a redraw method, or a fusion method. Adisk-shaped glass substrate, which is the base of a magnetic-disk glasssubstrate, can be obtained by appropriately performing shape processingon the plate-shaped glass blank produced using these known manufacturingmethods.

(b) Rough Grinding

In rough grinding, main surfaces on both sides of the glass blank areground. Loose abrasive particles are used as an abrasive material, forexample. In rough grinding, grinding is performed such that the glassblank is brought approximately closer to a target substrate thicknessand a target flatness of the main surfaces. Note that rough grinding isperformed according to the dimensional accuracy or the surface roughnessof the molded glass blank, and may be omitted in some cases.

(c) Shape Processing

Next, shape processing is performed. In the shape processing, after theglass blank is molded, a circular hole is formed using a knownprocessing method to obtain a disk-shaped glass substrate having acircular hole. Then, chamfering of edge surfaces of the glass substrate,and adjusting of the outer diameter and the inner diameter are carriedout through grinding processing. Accordingly, a side wall surface 3 borthogonal to the main surfaces and chamfered surfaces 3 c that areinclined with respect to the main surfaces 3 a and between the side wallsurface 3 b and the main surfaces 3 a on both sides are formed on theedge surfaces of the glass substrate.

(d) Edge Surface Polishing

Next, edge surface polishing is performed on the glass substrate. Edgesurface polishing is processing for performing polishing by supplying apolishing liquid containing loose abrasive particles between a polishingbrush and the edge surfaces (the side wall surface 3 b and the chamferedsurfaces 3 c) of the glass substrate and moving the polishing brush andthe glass substrate relative to each other. In edge surface polishing,an inner circumferential side edge surface and an outer circumferentialside edge surface of the glass substrate are polishing targets, and theinner circumferential side edge surface and the outer circumferentialside edge surface are formed into mirror surfaces. Note that edgesurface polishing need not be performed in some cases.

(e) Precision Grinding

Next, precision grinding is performed on the main surfaces of the glasssubstrate. For example, the main surfaces 3 a of the glass substrate areground using a double-side grinding apparatus provided with a planetarygear mechanism. In this case, grinding is performed with the surfaceplates provided with fixed abrasive particles, for example.Alternatively, grinding is also performed using loose abrasiveparticles. Note that precision grinding need not be performed in somecases.

(1) First Polishing

Next, first polishing is performed on the main surfaces 3 a of the glasssubstrate. First polishing is performed using loose abrasive particlesand polishing pads attached to the surface plates. First polishingremoves cracks and warping remaining on the main surfaces 3 a in thecase where grinding is performed with fixed abrasive particles, forexample. In first polishing, surface roughness of the main surfaces 3 a,or for example, an arithmetic average roughness Ra, can be reduced whilepreventing the shape of the edge portions of the main surfaces 3 a frombeing excessively recessed or protruding.

Although there is no particular limitation on the loose abrasiveparticles used in first polishing, cerium oxide abrasive particles,zirconia abrasive particles, or the like are used, for example. Notethat first polishing need not be performed in some cases.

(g) Chemical Strengthening

The glass substrate can be chemically strengthened as appropriate. Amelt obtained by heating potassium nitrate, sodium nitrate, or a mixturethereof, for example, is used as a chemical strengthening liquid. Also,by immersing the glass substrate in the chemical strengthening liquid,lithium ions and sodium ions in the glass composition that are presentin a surface layer of the glass substrate are respectively substitutedwith sodium ions and potassium ions in the chemical strengthening liquidwhose ion radii are relatively large, whereby compressive stress layersare formed on the surface layer portions and the glass substrate isstrengthened.

Although the timing at which chemical strengthening is performed isdetermined as appropriate, the polishing is particularly preferablyperformed after chemical strengthening, because the surface can besmoothed and foreign matter attached to the surface of the glasssubstrate can be removed through chemical strengthening. Also, chemicalstrengthening need not be performed in some cases.

(h) Second Polishing (Mirror-Polishing)

Next, second polishing is performed on the chemically strengthened glasssubstrate. Second polishing is for performing mirror-polishing on themain surfaces 3 a. In second polishing as well, polishing is performedusing a polishing apparatus having a configuration similar to that infirst polishing. In second polishing, the type and the particle size ofloose abrasive particles are changed relative to first polishing andmirror polishing is performed using resin polishers having a lowhardness as the polishing pads. Doing so makes it possible to reduce theroughness of the main surfaces 3 a while preventing the shape of edgeportions of the main surfaces 3 a from being excessively recessed orprotruding. The main surfaces 3 a preferably have an arithmetic averageRa (JIS B 0601 2001) of 0.2 nm or less. Note that second polishing neednot be performed in some cases because the main surfaces 3 a of thesubstrate that have been subjected to second polishing are not theoutermost surface of the non-magnetic substrate 1 having the film 4.

(i) Film Formation

The film 4 is formed on the main surfaces 3 a, the side wall surface 3b, and the chamfered surfaces 3 c of the produced glass substrate. Thefilm 4 is formed through electrolytic plating, electroless plating, orthe like. Pre-treatment for improving the adherence of the film 4 may beperformed as needed before the film 4 is formed. The film 4 is formed onthe main surfaces 3 a, the side wall surface 3 b, and the chamferedsurfaces 3 c, and can have the same thickness on any of the surfaces. Inorder to reduce internal stress of the formed film 4, annealing (heattreatment) is performed on the film 4 as needed after the film 4 isformed. Note that the film 4 is preferably a non-magnetic film so as notto cause noise when a magnetic disk is finally produced.

(j) Film Polishing

After the film 4 is formed, in order to reduce surface roughness of thefilm 4, the film 4 provided on the main surfaces 3 a of the substratemain body 3 are polished. Film polishing aims to realizemirror-polishing. The main surfaces that have been subjected to filmpolishing preferably have an arithmetic average roughness Ra of 0.2 nmor less, the arithmetic average roughness being measured using an atomicforce microscope (AFM). In film polishing as well, polishing can beperformed using a polishing apparatus having a configuration similar tothat in first polishing. In film polishing, the type and the particlesize of loose abrasive particles are changed relative to first polishingand polishing is performed using resin polishers having a low hardnessas the polishing pads. In film polishing, polishing may be performed aplurality of times as needed. In this case, precise polishing isperformed using loose abrasive particles with a smaller size inpolishing in the downstream processes. In this manner, the film 4 formedon the main surfaces 3 a is polished, and the film 4 formed on the sidewall surface 3 b and the chamfered surfaces 3 c are not polished, andthus the film 4 formed on the side wall surface 3 b and the chamferedsurfaces 3 c can be made thicker than the film 4 on the main surfaces 3a.

As described above, because the film 4 formed on the side wall surface 3b and the chamfered surfaces 3 c exhibits the effect of suppressing theamplitude of vibration of the main surfaces 3 a of the non-magneticsubstrate 1, it is preferable to set the thickness of the film 4 to beformed such that the thickness of the film 4 formed on the side wallsurface 3 b and the chamfered surfaces 3 c has a thickness to an extentthat the amplitude of vibration of the main surfaces 3 a can besuppressed.

After film polishing is performed, the glass substrate provided with thefilm 4 is cleaned to produce a non-magnetic substrate 1 for a magneticdisk.

Note that, if the substrate main body 3 is an AL alloy substrate, thesubstrate main body 3 is produced using the following method, forexample.

First, an Al alloy substrate, which is to be the substrate main body 3,is subjected to machining to have a shape with a predetermined sizethrough cutting. In order to improve shape accuracy and flatness of thesubstrate main body 3, hot-press annealing is then performed. Also, theedge surfaces (inner and outer circumferential edge surfaces) of thesubstrate main body 3 are ground and polished. When edge surfaces areground, the edge surfaces of the substrate main body 3 are ground byrotating the edge surfaces and a rotation tool, and supplying a grindingliquid from a nozzle while pressing the rotation tool to which abrasiveparticles are fixed against edge surfaces of the substrate main body 3that has been cut in a manner similar to that for the glass substrate.Also, if surface roughness of an edge surface is to be reduced, apolisher made of nonwoven fabric is attached to the surface of therotation tool, and the edge surfaces of the substrate main body 3 arepolished while supplying a polishing liquid in which loose abrasiveparticles are dispersed. Also, the chamfered surfaces of the substratemain body 3 are formed through grinding using a formed grindstoneobtained by shaping an end portion of the rotation tool to a chamferingshape in advance. Next, the main surfaces of the substrate main body 3are ground using a double-side grinding apparatus, are polished aplurality of times using a double-side polishing apparatus,polyurethane-foam resin polishers, and a polishing liquid containingalumina abrasive particles or colloidal silica abrasive particles, andare lastly cleaned.

Note that zincate treatment may be performed on the substrate main body3 as pre-treatment of formation of the film 4. After the film 4 isformed, annealing is performed as appropriate in order to reduce theinternal stress of the film 4. After the film 4 is annealed, the mainsurfaces 3 a are polished. Polishing is performed a plurality of timesas needed for the substrate. Then, cleaning is performed to produce anon-magnetic substrate 1 for a magnetic disk.

In addition to the above-described Ni—P alloy, an Mg alloy, an Al—Znalloy, an Mg—Zr alloy, and the like can be used for the film 4. Here,from the viewpoint of suppressing vibration of the non-magneticsubstrate 1, the material of the film 4 has a higher loss factor thanthe substrate main body 3 and has a loss factor of 0.01 or more, andaccording to one embodiment, the material of the film 4 preferably has aloss factor of 0.02 or more, and more preferably has a loss factor of0.03 or more. Also, the substrate main body 3 preferably has a lossfactor of 0.002 or less, and more preferably has a loss factor of 0.001or less. The smaller the loss factor of the substrate main body 3 is,the better the vibration suppression effect of the film 4 is, and thusthe substrate main body 3 having a smaller loss factor is preferable. Anamorphous aluminosilicate glass substrate to be used for a magnetic diskhas a loss factor of 0.001 or less, for example. Also, an Al—Mg alloysubstrate for a magnetic disk has a loss factor of 0.002 or less, forexample. In this manner, the loss factor of the film 4 is sufficientlylarge with respect to the loss factor of the substrate main body 3, andthus, as a result of forming the film 4, a vibration suppression effectcan be effectively obtained.

Also, according to one embodiment, from the viewpoint that particles,scratches, and recesses are less likely to occur, the material of thefilm 4 preferably has a Vickers hardness Hv of 100 [kgf/mm²] or more,more preferably has a Vickers hardness Hv of 200 [kgf/mm²] or more, andeven more preferably has a Vickers hardness Hv of 400 [kgf/mm²] or more.

Table 1 below shows properties of materials that can be suitably used asthe material of the substrate main body 3 and the film 4. Loss factorsshown in Table 1 below were calculated using the above-described lossfactor calculation method. The value of a loss factor is the value atroom temperature. A Vickers hardness Hv is measured using amicro-Vickers hardness tester, under conditions in which the indenterload was 10 gf for the film 4 because the film 4 was a thick film andthe indenter load was 300 gf for the substrate main body 3.

TABLE 1 Vickers Hardness Loss Factor [kgf/mm²] Aluminosilicate0.0003~0.0008 650 glass Al—Mg Alloy 0.0005~0.0017 50~100 Mg Alloy 0.01 55 Al—Zn Alloy 0.05 100 Mg—Zr Alloy 0.09 100 Ni—P Alloy 0.03 500

In Table 1 above, the loss factor of aluminosilicate glass is obtainedfrom glass having the composition of the above-described glass 1 orglass 2. Note that the glass 2 typically has a loss factor of 0.0006.

The composition of the Al—Mg alloy includes Mg in an amount of 3.5 to 5mass %, Si in an amount of 0 to 0.05 mass %, Fe in an amount of 0 to 0.1mass %, Cu in an amount of 0 to 0.12 mass %, Mn in an amount of 0 to 0.3mass %, Cr in an amount of 0 to 0.1 mass %, Zn in an amount of 0 to 0.5mass %, Ti in an amount of 0 to 0.1 mass %, and Al as the remainingportion, for example.

The composition of an Mg alloy includes Mg in an amount of 91.57 mass %,Al in an amount of 7.6 mass %, Zn in an amount of 0.7 mass %, and Mn inan amount of 0.13 mass %.

The composition of an Al—Zn alloy includes Al in an amount of 60 mass %and Zn in an amount of 40 mass %.

The composition of an Mg—Zr alloy include Mg in an amount of 99.4 mass %and Zn in an amount of 0.6 mass %.

The composition of an Ni—P alloy includes Ni in an amount of 90 mass %and P in an amount of 10 mass %.

As is understood from Table 1, from the viewpoint of vibrationsuppression, it is preferable to use aluminosilicate aluminosilicateglass or an Al—Mg alloy as the material of the substrate main body 3,and use an Ni—P alloy, an Mg alloy, an Al—Zn alloy, and an Mg—Zr alloyas the material of the film 4. Also, an Ni—P alloy, an Al—Zn alloy, andan Mg—Zr alloy have a high Vickers hardness Hv of 100 [kgf/mm²] or more,and are more suitable as the material of the film 4, and particles,scratches, and recesses are less likely to occur. Also, it is understoodthat an Ni—P alloy has a very high Vickers hardness Hv, and is even moresuitable as the material of the film 4.

Experimental Example 1

In order to examine the effects of the non-magnetic substrate 1, varioussubstrates were produced.

Amorphous aluminosilicate glass and an aluminum alloy (Al—Mg alloy)satisfying the above-described glass composition was used as thematerial of the substrate main body of the non-magnetic substrate 1. Ina case where the film 4 was formed, the film 4 was formed throughelectroless plating such that an Ni—P alloy (P: 10 mass %, the remainingportion was Ni) covered all of the surfaces of the substrate main body 3at an equal film thickness. Then, main surfaces on both sides werepolished using a double-side polishing apparatus, and 20% of thethickness of the film 4 formed on each main surface was removed throughpolishing. The film 4 had the same final thickness on the main surfaceson both sides, and the thickness of the film 4 formed on each mainsurface was 80% of the thickness of the film 4 formed on an edgesurface. The loss factor of the Ni—P alloy satisfied 0.01 or more.

Also, the produced non-magnetic substrate 1 had an outer diameter of 95mm and an inner diameter (the diameter of a circular hole) of 25 mm, andchamfered surfaces were formed at connection portions of an outercircumferential edge surface and an inner circumferential edge surfacethat are respectively connected to both main surfaces. With regard tothe specification of this chamfered surface, an angle to a main surfacewas 45 degrees, the length thereof in a radial direction was 150 μm, andthe length thereof in the thickness direction was 150 μm. Roughness ofthe outer circumferential edge surface at this time was adjusted suchthat the surface of the substrate main body 3 and the surface of thenon-magnetic substrate 1 had a maximum height Rz of 0.1 μm.

A magnetic disk obtained by forming a magnetic film on the producednon-magnetic substrate 1 was incorporated in an evaluation apparatusobtained by modifying a hard disk drive apparatus. The evaluationapparatus internally includes a high-speed camera for observingvibration of the magnetic disk, and can apply vibration of any magnitudeto the magnetic disk while observing vibration of the magnetic diskusing the high-speed camera. Note that the magnetic disk does not rotatein the evaluation apparatus, and evaluation is made in a stationarystate. A member imitating a ramp member produced using a PEEK (polyetherether ketone) was incorporated in the evaluation apparatus, and whenmagnetic disks were mounted, there were 0.2 mm-gaps extending from bothmain surfaces. That is, a gap between ramps that a magnetic disk entershad a length of the thickness of the magnetic disk+0.4 mm. A pluralityof evaluation apparatuses having different specifications were preparedso as to have a fixed gap, even if the thickness of a substrate waschanged. Note that, even if the thickness of a magnetic film formed in amedia process can be substantially ignored because the thickness of themagnetic film was about 100 nm or less even if it includes a base filmand a soft magnetic layer. Five magnetic disks were mounted in theevaluation apparatus, and a portion of main surfaces of all magneticdisks were held by the ramp member.

An impact test for applying a constant impact to the magnetic disksusing the evaluation apparatus was performed. The magnitude of theimpact was optimized to be a value at which a magnetic disk of acomparative example, which was the standard (100%) of a defect/foreignmatter ratio in each of Tables 2 to 5, collided with the ramp severaltimes. Specifically, in the case of Table 2, the impact was appliedunder the conditions of 140 G and 2 m seconds. Hereinafter, the impactwas 120 G in Table 3, 90 G in Table 4, and 70 G in Table 5 with theapplication time kept at 2 m seconds. After the impact test wasperformed, the number of particles and defects such as scratches andrecesses on main surfaces of 5 incorporated magnetic disks were countedby visually examining the surfaces thereof under a condensing lamp in adark room. These particles, scratches, and recesses occurred as a resultof the magnetic disks coming into contact with the ramp member and otherdisks in the apparatus in the impact test. In Tables 2 to 5 below, thenumber of particles, scratches, and recesses in examples and comparativeexamples was shown as relative “defect and foreign matter ratios” wherethe number of particles, scratches, and recesses in Comparative Examples1, 4, 7, or 10 was the standard (100%). The smaller the defect andforeign matter ratio is, the smaller the number of particles, scratches,and recesses is. It is inferred that, the smaller the defect and foreignmatter ratio is, the longer the lifespan of the hard disk driveapparatus substantially is due to the effect of suppressing particlesetc.

In the comparative examples and examples shown in Tables 2 to 5 below,the ratio D/T was changed in various ways by changing the thickness ofthe film 4 with the thickness of the substrate main body 3 keptconstant.

TABLE 2 Conv. Ex. 1 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Ex. 1 Ex. 2 Ex.3 Ex. 4 Material of Substrate glass glass glass glass glass glass glassglass Main Body 3 Presence of Film 4 no no yes yes yes yes yes yesThickness of Non- 0.750 0.640 0.640 0.640 0.640 0.640 0.640 0.640Magnetic Substrate 1 (mm) Ratio D/T — — 0.01 0.02 0.025 0.03 0.04 0.05Defect and Foreign 0 100 95 90 68 43 10 7 Matter Ratio (%) (standard)

TABLE 3 Conv. Ex. 2 Comp. Ex. 4 Comp. Ex. 5 Comp. Ex. 6 Ex. 5 Ex. 6 Ex.7 Ex. 8 Material Of Substrate aluminum aluminum aluminum aluminumaluminum aluminum aluminum aluminum Main Body 3 alloy alloy alloy alloyalloy alloy alloy alloy Presence of Film 4 no no yes yes yes yes yes yesThickness of Non- 0.750 0.640 0.640 0.640 0.640 0.640 0.640 0.640Magnetic Substrate 1 (mm) Ratio D/T — — 0.01 0.02 0.025 0.03 0.04 0.05Defect and Foreign 0 100 98 92 58 31 7 4 Matter Ratio (%) (standard)

TABLE 4 Comp. Ex. 7 Comp. Ex. 8 Comp. Ex. 9 Ex. 9 Ex. 10 Ex. 11 Ex. 12Ex. 13 Material of Substrate glass glass glass glass glass glass glassglass Main Body 3 Presence of Film 4 no yes yes yes yes yes yes yesThickness of Non- 0.520 0.520 0.520 0.520 0.520 0.520 0.520 0.520Magnetic Substrate 1 (mm) Ratio D/T — 0.01 0.02 0.025 0.03 0.04 0.050.062 Defect and Foreign 100 93 88 54 26 8 6 4 Matter Ratio (%)(standard)

TABLE 5 Comp. Ex. Comp. Ex. Comp. Ex. 10 11 12 Ex. 14 Ex. 15 Ex. 16 Ex.17 Ex. 18 Material of Substrate aluminum aluminum aluminum aluminumaluminum aluminum alluminum aluminum Main Body 3 alloy alloy alloy alloyalloy alloy alloy alloy Presence of Film 4 no yes yes yes yes yes yesyes Thickness of Non- 0.520 0.520 0.520 0.520 0.520 0.520 0.520 0.520Magnetic Substrate 1 (mm) Ratio D/T — 0.01 0.02 0.025 0.03 0.04 0.050.062 Defect and Foreign 100 97 91 45 16 3 2 2 Matter Ratio (%)(standard)

According to Tables 2 and 3, although problems of particles, scratches,and recesses did not occur at all in Conventional Examples 1 and 2 inwhich the non-magnetic substrate 1 was thicker than 0.700 mm, problemsof particles, scratches, and recessed occurred in Comparative Examples 1and 3 in which the non-magnetic substrate 1 was thinner than 0.700 mm.However, through comparisons between Comparative Examples 2 and 3 andExamples 1 to 4, and comparisons between Comparative Examples 5 and 6and Examples 5 to 8, it was understood that the number of particles,scratches, and recesses was effectively reduced by setting the ratio D/Tto be 0.025 or more.

As shown in Tables 4 and 5, through comparisons between ComparativeExamples 8 and 9 and Examples 9 to 13, and comparisons betweenComparative Examples 11 and 12 and Examples 14 to 18, it is understoodthat, even if the thickness of the non-magnetic substrate 1 was 0.520mm, the number of particles, scratches, and recesses was effectivelyreduced by setting the ratio D/T to be 0.025 or more. Also, it isunderstood that the effect of reducing the number of particles,scratches, and recesses was better than in a case where the thickness ofthe non-magnetic substrate 1 was 0.640 mm.

Also, in Tables 2 to 5, in a case where examples (e.g., Examples 10 and15) having the same specifications except for the material of thesubstrate main body 3 are compared to each other, the material of thesubstrate main body 3 had a smaller defect and foreign matter ratio in acase where an Al alloy was used than in a case where glass was used.Accordingly, it can be said that vibration was further suppressed in acase where an Al alloy substrate was used as the substrate main body 3than in a case where glass was used as the substrate main body 3. Notethat the above-described vibration suppression effect is more effectivein a case where there is a high possibility that a magnetic disk willcome into contact with a ramp, and thus the non-magnetic substrate 1 isparticularly preferable in an HDD having a specification in which a gapbetween ramps that the magnetic disk enter has a length of “thethickness of the magnetic disk+0.4 mm or less”.

Experimental Example 2

In order to examine surface roughness of the substrate main body 3 at aninterface where the substrate main body 3 is in contact with the film 4and vibration suppression effect, various substrates were produced.

In the non-magnetic substrate 1 produced in Experimental Example 2, thespecifications of the substrate main body 3 and the film 4 were the sameas those in Experimental Example 1, except that surface roughness of theouter circumferential edge surface of the substrate main body 3 wasadjusted by changing conditions of edge surface grinding and edgesurface polishing as appropriate. A magnetic disk obtained by forming amagnetic film on the non-magnetic substrate 1 was incorporated in thesame evaluation apparatus as that of Experimental Example 1, and thevibration suppression effect was evaluated by performing the same impacttest as that of Experimental Example 1 and obtaining defect and foreignmatter ratios.

Examples 1, 5, and 14 shown in Tables 6 to 8 below were the same asExamples 1, 5, and 14 shown in Tables 2, 3, and 5. In those examples,the surface roughness maximum height Rz of the substrate main body 3 atthe interface where the substrate main body 3 is in contact with thefilm 4 was 0.1 μm.

The maximum height Rz of an outer circumferential edge surface wasadjusted based on Examples 1, 5, and 14 without changing the type ofsubstrate main body 3, whether or not the film 4 was present, thethickness of the non-magnetic substrate 1, and the ratio D/T. Themaximum height Rz was changed by changing the count of a grindinggrindstone and the period of time for polishing the outercircumferential edge surface in outer circumferential edge surface shapeprocessing at the time of production of the substrate main body 3. Thus,examples in which surface roughness of outer circumferential edgesurfaces was adjusted based on Example 1 were Examples 1A, 1B, and 1C inTable 6 below, and examples in which surface roughness was adjustedbased on Examples 5 and 14 were Examples 5A, 5B, and 5C and Examples14A, 14B, and 14C in Tables 7 and 8 in a similar manner.

TABLE 6 Ex. 1 Ex. 1A Ex. 1B Ex. 1C Max. Height Rz of Side Wall Surface0.1 0.5 1.0 1.5 of Substrate Main Body 3 at Interface where SubstrateMain body 3 is in Contact with Film 4 (μm) Defect and Foreign MatterRatio (%) 68 63 61 59

TABLE 7 Ex. 5 Ex. 5A Ex. 5B Ex. 5C Max. Height Rz of Side Wall Surface0.1 0.5 1.0 1.5 of Substrate Main Body 3 at Interface where SubstrateMain body 3 is in Contact with Film 4 (μm) Defect and Foreign MatterRatio (%) 58 53 51 49

TABLE 8 Ex. Ex. Ex. 14 Ex. 14A 14B 14C Max. Height Rz of Side WallSurface of 0.1 0.5 1.0 1.5 Substrate Main Body 3 at Interface whereSubstrate Main body 3 is in Contact with Film 4 (μm) Defect and ForeignMatter Ratio (%) 45 40 38 36

It is understood that, in any of the cases shown in Tables 6 to 8, thenumber of particles, scratches, and recesses was effectively reduced bysetting the maximum height Rz of the substrate main body 3 at theinterface where the substrate main body 3 is in contact with the film 4to be 0.5 μm or more, and those examples had a strong vibrationsuppression effect. Also, it is understood that a much better vibrationsuppression effect can be obtained by setting the maximum height Rz tobe 1.0 μm or more, or to be 1.5 μm or more.

From the above-described results of evaluation, the effects of thenon-magnetic substrate 1 for a magnetic disk are clear.

As described above, although a non-magnetic substrate for a magneticdisk and a magnetic disk according to the present invention have beendescribed in detail, the present invention is not limited to theabove-described embodiment and examples etc., and it will be appreciatedthat various improvements and modifications can be made withoutdeparting from the gist of the present invention.

The invention claimed is:
 1. A non-magnetic substrate for a magneticdisk, the non-magnetic substrate comprising: a substrate main bodyhaving two opposing main surfaces, the substrate main body being a glasssubstrate main body or an aluminum alloy substrate main body; and ametal film that is provided on the main surfaces and an outercircumferential edge surface of the substrate main body, and is made ofan Ni—P alloy, wherein the non-magnetic substrate has a thickness (T+D)of 0.640 mm or less, the thickness (T+D) being the sum of a thickness Tof the substrate main body and a thickness D of the metal film, a ratioD/T of the thickness D of the metal film to the thickness T of thesubstrate main body is 0.025 or more, and the outer circumferential edgesurface of the substrate main body forming an interface with the metalfilm has a surface roughness maximum height Rz of 0.5 μm or more.
 2. Thenon-magnetic substrate for a magnetic disk according to claim 1, whereinthe metal film is provided on each of the main surfaces, and is providedon the outer circumferential edge surface of the substrate main body,and the thickness of the metal film provided on the outercircumferential edge surface is greater than the thickness of the metalfilm provided on each of the main surfaces.
 3. The non-magneticsubstrate for a magnetic disk according to claim 2, wherein a surfaceroughness maximum height Rz of the metal film formed on the outercircumferential edge surface of the non-magnetic substrate for amagnetic disk is smaller than a surface roughness maximum height Rz ofthe substrate main body at the outer circumferential edge surface of thesubstrate main body.
 4. The non-magnetic substrate for a magnetic diskaccording to claim 3, wherein the non-magnetic substrate has a thicknessof 0.570 mm or less.
 5. The non-magnetic substrate for a magnetic diskaccording to claim 2, wherein the non-magnetic substrate has a thicknessof 0.570 mm or less.
 6. The non-magnetic substrate for a magnetic diskaccording to claim 1, wherein the non-magnetic substrate has a thicknessof 0.570 mm or less.
 7. The non-magnetic substrate for a magnetic diskaccording to claim 1, wherein the non-magnetic substrate has a thicknessof 0.520 mm or less.
 8. The non-magnetic substrate for a magnetic diskaccording to claim 1, wherein the non-magnetic substrate for a magneticdisk has a disk shape, and the disk shape has an outer diameter of 90 mmor more.
 9. The non-magnetic substrate for a magnetic disk according toclaim 1, wherein the metal film has a Vickers hardness Hv of 100(kgf/mm²) or more.
 10. A magnetic disk, wherein a surface of thenon-magnetic substrate for a magnetic disk according to claim 1 has atleast a magnetic film.
 11. The non-magnetic substrate for a magneticdisk according to claim 1, wherein a surface roughness maximum height Rzof the metal film formed on the outer circumferential edge surface ofthe non-magnetic substrate for a magnetic disk is smaller than a surfaceroughness maximum height Rz of the substrate main body at the outercircumferential edge surface of the substrate main body.
 12. Thenon-magnetic substrate for a magnetic disk according to claim 11,wherein the non-magnetic substrate has a thickness of 0.570 mm or less.13. A non-magnetic substrate for a magnetic disk, the non-magneticsubstrate comprising: a substrate main body having two opposing mainsurfaces, the substrate main body being a glass substrate main body oran aluminum alloy substrate main body; and a metal film that is providedon the main surfaces and an outer circumferential edge surface of thesubstrate main body, and is made of an Ni—P alloy, wherein thenon-magnetic substrate has a thickness (T+D) of 0.640 mm or less, thethickness (T+D) being the sum of a thickness T of the substrate mainbody and a thickness D of the metal film, a ratio D/T of the thickness Dof the metal film to the thickness T of the substrate main body is 0.025or more, and a surface roughness maximum height Rz of the metal filmformed on the outer circumferential edge surface of the non-magneticsubstrate for a magnetic disk is smaller than a surface roughnessmaximum height Rz of the substrate main body at the outercircumferential edge surface of the substrate main body.
 14. Thenon-magnetic substrate for a magnetic disk according to claim 13,wherein the metal film is provided on each of the main surfaces, and isprovided on the outer circumferential edge surface of the substrate mainbody, and the thickness of the metal film provided on the outercircumferential edge surface is greater than the thickness of the metalfilm provided on each of the main surfaces.
 15. The non-magneticsubstrate for a magnetic disk according to claim 14, wherein thenon-magnetic substrate has a thickness of 0.570 mm or less.
 16. Thenon-magnetic substrate for a magnetic disk according to claim 13,wherein the non-magnetic substrate has a thickness of 0.570 mm or less.17. The non-magnetic substrate for a magnetic disk according to claim13, wherein the non-magnetic substrate has a thickness of 0.520 mm orless.
 18. The non-magnetic substrate for a magnetic disk according toclaim 13, wherein the non-magnetic substrate for a magnetic disk has adisk shape, and the disk shape has an outer diameter of 90 mm or more.19. The non-magnetic substrate for a magnetic disk according to claim13, wherein the metal film has a Vickers hardness Hv of 100 (kgf/mm²) ormore.
 20. A magnetic disk, wherein a surface of the non-magneticsubstrate for a magnetic disk according to claim 13 has at least amagnetic film.